1. Field of the Invention
The invention relates to a thin-film transistor array, a method of fabricating the same, and a liquid crystal display device including the same.
2. Description of the Related Art
A transparent electrically conductive film composed of oxide, such as an ITO film composed of compound of indium (In), tin (Sn), and oxygen (O), is frequently used in a flat panel display or a photoelectric transfer device, since it has a sheet resistance of a few ohms per a unit area even if it has a small thickness such as hundreds of nanometers, and it has high transmittance to visible light.
Furthermore, a study to a thin-film transistor including a channel layer composed of transparent oxide semiconductor such as In—Ga—Zn—O has been recently started.
Such oxide semiconductor contains highly ionic bonds, and is characterized by a small difference in electron mobility between crystalline state and amorphous state.
Accordingly, relatively high electron mobility can be obtained even in amorphous state.
Since an amorphous film of oxide semiconductor can be formed at room temperature by carrying out sputtering, a study about a thin-film transistor composed of oxide semiconductor to be formed on a resin substrate such as a PET substrate has been started.
For instance, Japanese Patent Application Publication No. 2003-50405 has suggested a thin-film transistor array including an oxide-semiconductor film formed on a substrate. By applying electrical conductivity to a certain part of the oxide-semiconductor film, there are formed a channel, a source, and a drain of a thin-film transistor, and there are further formed an electrically conductive part including a pixel electrode electrically connected to the drain.
In the above-mentioned thin-film transistor array, a part composed of intrinsic oxide-semiconductor containing no impurities defines a channel of a thin-film transistor, and an electrically conductive part into which impurities were doped defines both a source and a drain of a thin-film transistor, and a pixel electrode.
However, the above-mentioned thin-film transistor array is accompanied with a problem that it is unavoidable for a number of carrying out a photolithography step to increase, since an electrically conductive part is formed by doping impurities thereinto through the use of a photomask.
Furthermore, the above-mentioned Publication is silent about a signal line terminal through which a thin-film transistor is electrically connected to an external drive circuit.
Accordingly, there is not accomplished so far a thin-film transistor array composed of oxide semiconductor, which is capable of ensuring electrical connection between a signal line terminal and an external driver circuit without an increase in a number of carrying out a photolithography step.
Japanese Patent Application Publication No. 10-31227 (published on February 1998) has suggested a thin-film transistor array substrate including a plurality of display units each comprised of a top gate type thin-film transistor, and a pixel electrode electrically connected to a drain electrode. A source electrode, a drain electrode, a pixel electrode, a signal line, a signal line terminal, and a scanning line terminal are comprised commonly of a transparent electrically conductive film. The transparent electrically conductive film is exposed outside at both the signal line terminal and the scanning line terminal through a terminal contact hole. A refractive metal film is formed at a boundary between portions of the transparent electrically conductive film located at the signal line terminal and the scanning line terminal, and an inner wall of a contact hole formed throughout an electrically insulating film. The refractive film is covered with an inorganic electrically insulating film.